Fig. 4 XPS-derived Pd surface content vs. bulk composition on 0.3 wt%.
PdxPt1Àx/TiO2, showing the absence of any surface segregation.
Fig. 5 Simultaneous CO and acetone photooxidation activities on
0.3 wt% PdxPt1Àx/TiO2 vs. Pd composition (%) at 0% RH (brown) and
50% RH (white). [CO]inlet = 250 ppm and [acetone]inlet = 1800 ppm.
The association of Pt and Pd atoms leads to PdPt bimetallic
particles with new, i.e. weaker adsorption properties,
especially for compositions in the 30–50% Pd range. These
new adsorption properties explain the improved conversion rates
under 50% RH. The synergy between Pd and Pt comes from a
redistribution of the d-electrons in the bimetallic particles. The
formation of hetero-metallic bonds, the modification of M–M
distances and the orbital overlap imply band rehybridisation.
As a consequence of this rehybridisation,20 the population of
the sp band increases to strengthen the M–M bonds at the expense
of the d band, which is depleted and whose center is driven away
from the Fermi level.21–23 The adsorption properties of PdPt
bimetallicparticlesarethusmodified. LuoandFutamatareported
that water does not adsorb on PdPt particles in the presence of
CO.24 Therefore, in our case, PdPt particles remain water-free,
which explains the high stability to water vapor obtained for
bimetallic particles containing 30 to 50% of Pd, for which the
adsorption properties are modified by the largest amount.
Another important effect due to the d-band center shift is
the weaker adsorption of CO on the PdxPt1Àx bimetallic
particles.21–23 This enhances the reactivity of CO.
This critical point was, up to now, a limitation for the
elaboration of efficient indoor air treatment photocatalysts,
taking into account the unavoidable presence of CO and humid-
ity in indoor air environments. Furthermore, by contrast to
single M/TiO2 and metal-free TiO2 photocatalysts that showed
poor efficiencies in that case, bimetallic PdxPt1Àx based photo-
catalysts are also efficient for the simultaneous elimination of CO
and VOCs in the presence of humidity, illustrated in Fig. 5 for
CO and acetone removal. This provides a new hope for designing
efficient and safe indoor air purification photocatalytic devices.
The authors thank DGA (Direction Generale de l’Armement)
for financial support, A. Rach for technical support and
P. Bernhardt for XPS measurements (LMSPC) as well as
Dr C. Ulhaq (IPCMS) for HRTEM analysis.
Notes and references
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It must be underlined that the aforementioned beneficial
effect of the formation of bimetallic PdxPt1Àx particles for CO
photooxidation in the presence of humidity is optimal for a
perfect and homogeneous mixing of Pt and Pd atoms. In this
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c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 5331–5333 5333